The invention relates to a semiconductor device provided with a plurality of transistors including a power transistor. Particularly, the invention relates to prevention countermeasures for characteristic deterioration caused by interference between respective transistors.
FIG. 8 is a circuit diagram showing an example of a loudspeaker output circuit (a hybrid integrated circuit) for a general type of audio equipment. The loudspeaker output circuit includes, as shown in FIG. 8, a pair of complementary transistors. The transistors are connected to a pair of loudspeakers in series. Specifically, a circuit including an NPN transistor 32A and a PNP transistor 33A is connected to a loudspeaker 31A. A circuit including an NPN transistor 32B and a PNP transistor 33B is connected to a loudspeaker 31B.
In such a loudspeaker output circuit, the output characteristics of the pair of loudspeakers are required to be identical. Generally, in a case where a plurality of circuits have respective power transistors, the plurality of power transistors are individually connected to and mounted on a substrate. If a plurality of power transistors are mounted onto a common substrate, deterioration in thermal characteristics and high voltage durability of the respective transistors which constitute the circuits may disadvantageously be caused by influences of current leakage between the respective transistors and heat generation. Accordingly, in a case where a plurality of circuits having identical output characteristics are required as in the example, transistors the number of which is equal to the number of the circuits which are required to have identical output characteristics are selected from among transistors which are individually produced. An NPN transistor and a PNP transistor having matched characteristics are connected in series. Thus, the plurality of circuits having identical characteristics are conventionally realized.
As disclosed in Japanese Laid-Open Patent Publication No. 6-342876, as a semiconductor device provided with a function for protecting an output transistor formed on a semiconductor substrate from a thermal breakdown, a semiconductor device in which a transistor for detecting a temperature of the output transistor is disposed in an area enclosed by the output transistor is proposed. In the semiconductor device, the output transistor and the transistor for detecting a temperature through which a weak signal flows are electrically separated by a single impurity diffusion region (a channel stopper) formed simultaneously with an emitter region of the transistor.
In recent years, high power and multi channel are required for an audio output amplifier. In a case where the above-described conventional loudspeaker circuit is used, transistors which are individually produced are used for constituting respective circuits. Therefore, it is necessary to match the electrical characteristics or characteristics such as thermal characteristics in the respective transistors in order to realize identical output characteristics of the plurality of circuits.
However, it is difficult to match the characteristics of transistors which are individually produced in accordance with the purposes, as described in the conventional technique. This causes an increase in the production cost. It is also difficult to obtain a plurality of circuits having sufficiently identical characteristics.
Moreover, there exists a problem in that transistors are individually connected in a circuit used for audio loudspeaker output, so that the size of the hybrid integrated circuit is increased.
In the semiconductor device including the output transistor having the thermal breakdown protecting function disclosed in the above-identified publication, insufficient thermal separation of the output transistor from the transistor for detecting a temperature which are separated by the channel stopper causes a problem in that the operation of one transistor is easily affected by another transistor. Especially, the operation of the transistor for detecting a temperature is easily affected by the output transistor and noise from the outside of the semiconductor device.